The present invention pertains generally to loudspeaker cones, and more particularly to method for making a novel loudspeaker cone assembly which has an integral surround.
Loudspeaker cones (or loudspeaker diaphragms) are well known. In the prior art, a fibrous or felted paper-type cone or diaphragm is adapted to be supported on a support frame through an annular suspension member (or rim). This annular member is also known as a surround. The surround is secured to the outer marginal edge of the cone and has predetermined elastic or flexibility properties. The surround may be formed as an integral generally radially directed extension of the periphery of the cone or diaphragm, as disclosed in U.S. Pat. Nos. 1,872,583, 2,302,178, and 4,646,874, or may be formed as a separate annular supporting flange or rim affixed to an outer peripheral edge of the cone, such as disclosed in U.S. Pat. Nos. 2,840,177 and 3,634,052. Alternatively, the loudspeaker cone or diaphragm may be formed of plastic material such as disclosed in U.S. Pat. No. 3,858,680.
With the advent of relatively sophisticated sound reproduction systems, such as in stereophonic sound systems, the dynamic or moving coil-type loudspeaker cones used for the mid and low frequency ranges are subjected to relatively high amplitude vibrations in the axial direction of the cone. Movement along the cone axis is called excursion; modern woofers, in particular, may be designed for very long excursions. Excursions of two centimeters are possible. In the past, many manufacturers of larger size loudspeaker cones have made the suspension rims, or surrounds, from rubber in order to achieve desired flexibility and durability. A drawback to the use of rubber suspension rims is their relatively high cost.
To overcome the higher cost of rubber speaker cone suspension rims, attempts have been made to manufacture the suspension rims from plastic foam material. One method for making plastic foam-type suspension rims entails cutting a bulk block of raw foam material into slices or sheets of predetermined thickness. A foam sheet is then placed between heated dies which form an annular rim after which the rim is affixed to the outer edge of a cone, generally by means of an adhesive or sewing.
This technique has not been totally successful because the process involves production of large quantities of excess cut foam pieces which must be disposed of as scrap. Another drawback is that the cut foam rims generally have insufficient strength and structural integrity to support larger diameter speaker cones. A suspension rim made from a double layer of cut foam sheets in a laminated construction by a similar heated die process was found to provide improved suspension rims, however.
Cut foam suspension rims also are given to deterioration over time, this is known as "foam rot". Foam rot is accelerated by sunlight, high humidity or high temperatures.
A drawback to all known prior methods or techniques for making loudspeaker cone suspension rims is that they do not facilitate economical manufacture of rims in small lots of different colors, as may be desired for various applications.
Thus, a need exists for a loudspeaker cone assembly having a suspension rim which exhibits desired strength, flexibility and durability and which is economical to manufacture. Further, a relatively low cost suspension rim which was high strength adhesion to the rim of the speaker cone and which can be economically manufactured with various surface colors would find wide acceptance.
In the prior art, injection molding of foam onto a loudspeaker cone has been discussed as a means to mitigate some of these shortcomings. The applicant has discovered that injection molding is not suitable for loudspeaker surround applications, however, since polyurethane foam is excessively viscous and sticky for injection molding at the thicknesses required for a loudspeaker cone surround.